System and Method for Adaptive Transmission Time Interval (TTI) Structure

ABSTRACT

Methods and devices are provided for communicating data in a wireless channel. In one example, a method includes adapting the transmission time interval (TTI) length of transport container for transmitting data in accordance with a criteria. The criteria may include (but is not limited to) a latency requirement of the data, a buffer size associated with the data, a mobility characteristic of a device that will receive the data. The TTI lengths may be manipulated for a variety of reasons, such as for reducing overhead, satisfy quality of service (QoS) requirements, maximize network throughput, etc. In some embodiments, TTIs having different TTI lengths may be carried in a common radio frame. In other embodiments, the wireless channel may partitioned into multiple bands each of which carrying (exclusively or otherwise) TTIs having a certain TTI length.

This patent application is a continuation of U.S. Non-Provisionalapplication Ser. No. 13/611,823, filed on Sep. 12, 2012 and entitled“System and Method for Adaptive Transmission Time Interval (TTI)Structure,”which is hereby incorporated by reference herein as ifreproduced in its entirety.

TECHNICAL FIELD

The present invention relates generally to wireless communications, andmore specifically, to a system and method for adapting the length oftransmission time intervals (TTIs).

BACKGROUND

Modern wireless networks must support the communication of diversetraffic types (e.g., voice, data, etc.) having different latencyrequirements, while at the same time satisfying overall network/channelthroughput requirements. The ability to satisfy these latency andthroughput requirements is affected by, inter alia, wireless channelconditions and wireless channel parameters. One wireless channelparameter that significantly affects both latency and throughputperformance is the size (or length) of the transport containers used tocarry the traffic. Conventional networks use a single, fixed-length,transport container, and are therefore limited in their ability to adaptto changes in wireless channel conditions, usage, etc.

SUMMARY OF THE INVENTION

Technical advantages are generally achieved by embodiments of thepresent invention which adapt the length of downlink transmission timeintervals (TTIs) in downlink radio frames to satisfy latency and/orthroughput performance.

In accordance with an embodiment, a method of communicating data in awireless channel is provided. In this example, the method comprisesreceiving a first data and a second data. The method further includestransporting the first data in transmission time intervals (TTIs) of thewireless channel having a first TTI length; and transporting the seconddata in TTIs of the wireless channel having a second TTI length that isdifferent than the first TTI length. A transmitting device forperforming this method is also provided. A device for receiving datatransmitted in accordance with this method is also provided.

In accordance with another embodiment, another method for communicatingdata in a wireless channel is provided. In this example, the methodincludes receiving a first data destined for a receiving device,selecting a first TTI length for transporting the first data, andtransmitting the first data in a first TTI of the wireless channelhaving the first TTI length. The method further includes receiving asecond data destined for the receiving device, selecting a second TTIlength for transporting the second data, and transmitting the seconddata in a second TTI of the wireless channel having the second TTIlength. A transmitting device for performing this method is alsoprovided. A device for receiving data transmitted in accordance withthis method is also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a diagram of an embodiment of a wirelesscommunications network;

FIG. 2 illustrates a diagram of a prior art downlink channel carryingfixed-length TTIs;

FIG. 3 illustrates a diagram of an embodiment of a downlink channelcarrying variable-length TTIs;

FIG. 4 illustrates a flowchart of an embodiment method for adaptingTTI-lengths in a DL channel;

FIG. 5 illustrates a diagram of an embodiment for selecting TTI-lengthsfor transporting data in a DL channel;

FIG. 6 illustrates a protocol diagram of an embodiment communicationsequence for adapting TTI-lengths in a DL channel; and

FIG. 7 illustrates a flowchart of another embodiment method for adaptingTTI-lengths in a DL channel;

FIG. 8 illustrates a protocol diagram of another embodimentcommunication sequence for adapting TTI-lengths in a DL channel;

FIG. 9 illustrates a diagram of another embodiment of a DL channelcarrying variable-length TTIs;

FIG. 10 illustrates a diagram of an embodiment of a DL channel carryingTTIs have various lengths;

FIG. 11 illustrates a diagram of an embodiment of a DL channel carryingTTIs have various lengths; and

FIG. 12 illustrates a block diagram of an embodiment of a communicationsdevice.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the preferredembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

Conventional wireless networks use fixed length transport containers.For instance, networks operating under the third generation partnership(3GGP) long term evolution (LTE) release eight (rel-8) telecommunicationstandards use one millisecond (ms) transmission time intervals (TTIs).The length of a transport container can significantly affect latencyperformance and throughput performance of the network. Specifically,shorter transport containers achieve superior latency performance byproviding more frequent transmission opportunities, while longertransport containers achieve superior throughput performance by reducingsignaling overhead. Hence, fixed length transport containers may beunable to satisfy latency requirements and/or provide desired throughputperformance under some network conditions. As such, mechanisms ortechniques for varying transport container length are desired in orderto achieve improved network performance.

Aspects of this disclosure provide mechanisms for adapting the length oftransport containers in accordance with various parameters (e.g.,latency requirements, buffer size, user mobility characteristics, etc.).Although much of this disclosure is presented in the context of LTE(e.g., transport containers may be referred to as TTIs, etc.), thetechniques and/or mechanisms discussed herein can be applied to non-LTEnetworks (e.g., any frequency division duplex and/or time divisionduplex communication systems). Although much of this disclosure arediscussed in the context of downlink communications, the principlesdescribed herein can also be applied to provide adaptive TTI structuresin uplink communications, as well as other forms of wirelesscommunications (e.g., device-to-device, etc).

FIG. 1 illustrates a wireless network 100 comprising a cellular coveragearea 101 within which an eNB 110 provides wireless access to a pluralityof UEs 115, 125. The eNB 110 may provide wireless access by establishinga downlink communication channel (solid arrows) and an uplinkcommunication channel (dotted arrows) with the UEs 115,125. In anembodiment, the wireless network 100 may operate in accordance with anLTE communication protocol. The downlink communication channel may carrydata channels (e.g., physical downlink shared channel (PDSCH), etc.) andcontrol channels (e.g., a physical downlink shared channels (PDCCH),etc.). More specifically, the control channels may include UE/groupspecific control channels and common control channels that carrydownlink control information to the UEs (and/or relays), as well asuplink (UL)-related control channels that carry various uplink controlinformation to the UEs (e.g., hybrid automatic repeat request (HARM),acknowledge/negative-acknowledge (ACK/NACK), UL grant etc.).

FIG. 2 illustrates a prior art DL channel 200 carrying a plurality ofradio frames 210-220. As shown, TTIs in the radio frames 210-220 arefixed length, with each TTI carrying a common control channel, agroup/UE-specific control channel, and UL-related control channels.

FIG. 3 illustrates an embodiment of a DL channel 300 carrying aplurality of radio frames 310-320. Unlike the prior art DL channel 200,the DL channel 300 carries variable-length TTIs. The periodicity of thecommon control channel is determined by the periodicity of the radioframes (e.g., one common control channel per radio-frame). Theperiodicity of the group/UE-specific control channel is determined bythe periodicity of variable-length TTIs (e.g., one group/UE-specificcontrol channel per TTI). Notably, including a group/UE-specific controlchannel in each TTI allows the eNB to dynamically schedule UEs to TTIsas often as the smallest length-TTI (i.e., as often as the atomicinterval). Further, the UL-related control channel is decoupled from theTTI structure, such that the periodicity of the UL-related controlchannel is independent from the length/periodicity of thevariable-length TTIs. For instance, the TTI 311 carries one UL-relatedcontrol channel, while the TTI 313 carries three UL-related controlchannels. Notably, some TTIs do not carry any UL-related controlchannels. Hence, the amount of control overhead in the DL channel 300 isvariable, and depends on the periodicity of the UL-related controlchannel (e.g., as configured by the network administrator) as well asthe periodicity of the group/UE specific control channel (e.g., asdetermined by the TTI-length configurations of the radio frames310-320).

FIG. 4 illustrates a flowchart of a method 400 for adapting TTI-lengthsin a DL channel. The method 400 begins at step 410, where the eNBreceives a first data destined for a first user. Thereafter, the method400 proceeds to step 420, where the eNB receives a second data destinedfor a second user. The first data and the second data may be buffered inseparate buffers of the eNB. Thereafter, the method 400 proceeds to step430, where the eNB selects a first TTI-length for transporting the firstdata. This selection may be made in accordance with various selectioncriteria, including latency requirements, buffer size, mobilitycharacteristics of the first user, etc. Thereafter, the method 400proceeds to step 440, where the eNB selects a second TTI-length fortransporting the second data. Next, the method 400 proceeds to step 450,where the eNB transmits the first data in a first TTI having the firstTTI-length. Next, the method 400 proceeds to step 460, where the eNBtransmits the second data in a second TTI having the second TTI-length.The first data and the second data may be transmitted in a commonradio-frame.

FIG. 5 illustrates a flowchart of a method 500 for selecting TTI-lengthsfor transporting data in a DL channel. Notably, the method 500represents just one example for selecting TTI-lengths. Other examplesthat consider other factors and/or have more TTI-length designations mayalso be used to select TTI-lengths for data transmission. The method 500begins at step 510, where the eNB determines whether the latencyrequirement of the data (e.g., whether the data requires low latency),which may be determined in accordance with the traffic type of the data.For instance, some traffic types (e.g., voice, mobile gaming, etc.) mayrequire low levels of latency, while other traffic types (e.g.,messaging, email, etc.) may have less stringent latency requirements.

If the data requires low latency, then a short TTI-length 515 isselected to transport the data. If the data has a higher (i.e., lessstringent) latency requirement, then the method 500 proceeds to step520, where the eNB determines the buffer size used to store the data.Specifically, the buffer size of the data is indicative of the amount ofdata that needs to be transported. When large amounts of data need to betransported, then longer TTI-lengths may provide higher throughput ratesby minimizing overhead. However, large TTI-lengths may not be warrantedwhen only small amounts of data need to be transported. For instance, ifthere is not enough data to fill the long TTI, then a medium TTI-lengthmay be more efficient. If the data has a small buffer size, then amedium TTI-length 525 is selected. Otherwise, if the data has a largebuffer size, then the method 500 proceeds to step 530.

At step 530, the eNB determines whether the user has a low, medium, highor very-high mobility characteristic. A user's mobility characteristicmay correspond to a rate at which the user is moving. For instance,users that are moving at a higher rates of speed (e.g., a usercommunicating in a car) have higher mobility characteristics than usersmoving at comparatively lower rates of speed (e.g., a user walkingthrough a park). Notably, a user's mobility characteristic is highlycorrelated to wireless channel stability, as highly mobile usersexperience more volatile channel conditions than less mobile users.Moreover, wireless channel stability heavily influences the degree towhich link adaptation can be improved through more frequent channelestimation opportunities. That is, users having moderate to highmobility characteristics may achieve improved bit-rates when usingmedium TTI-lengths (or even short TTI-lengths) due to enhanced linkadaptation resulting from more frequent channel estimationopportunities. These higher bitrates may outweigh the overhead savingsof long TTI-lengths, and consequently may increase overall throughputfor those users. However, fast link adaptation capabilities may be lessbeneficial for stationary or slow moving users, as those usersexperience relatively stable channel conditions. As a result, lowmobility users may derive higher throughput by exploiting thelow-overhead nature of long TTI-lengths, rather than the faster linkadaptation capabilities derived from medium or low TTI-lengths. Inaddition, users that have very high mobility characteristics (e.g.,users moving at very-high rates of speed) may derive little or no gainfrom link adaptation, as channel conditions may be changing too quicklyto perform channel estimation with sufficient accuracy to improve thebit-rate. Hence, very-high mobility users may achieve higher throughputfrom long TTI-lengths. Referring once again to the method 500, if thedata is destined for a user having moderate to high mobility, then theeNB selects a medium TTI-length for transporting the data (at step 530).Alternatively, if the user has either low or very-high mobility, thenthe eNB selects a medium TTI-length for transporting the data (at step530). Notability, degrees of mobility (low, medium, high, and very high)may be relative to the network conditions and/or capabilities of thewireless communication devices.

FIG. 6 illustrates a protocol diagram for a communications sequence 600for communicating data in TTIs having varying TTI-lengths. Thecommunications sequence 600 begins when a first data (Data_(—)1) 610 anda second data (Data_(—)1) 615 destined for the UE1 115 and UE 125(respectively) are communicated from the backhaul network 130 to the eNB110. Upon reception, the eNB 110 determines which TTI-length totransport the Data_(—)1 610 and the Data_(—)1 615. The eNB 110communicates the TTI-lengths by sending a TTI length configuration(Data_(—)1) message 620 and a TTI length configuration (Data_(—)2)message 625 to the UEs 115 and 125 (respectively). Thereafter, the eNB110 communicates the Data_(—)1 610 and the Data_(—)2620 via the DL datatransmission (Data_(—)1) 630 and the DL data transmission (Data_(—)2)635. In an embodiment, the DL data transmission (Data_(—)1) 630 and theDL data transmission (Data_(—)2) 635 may be carried in different lengthTTIs of a common radio-frame.

FIG. 7 illustrates a flowchart of a method 700 for adapting TTI-lengthsin a DL channel. The method 700 begins at step 710, where the eNBreceives a first data destined for a user. Thereafter, the method 700proceeds to step 720, where the eNB selects a first TTI-length fortransporting the first data. Thereafter, the method 700 proceeds to step730, where the eNB transmits the first data in a first TTI having thefirst TTI-length. Next, the method 700 proceeds to step 740, where theeNB receives a second data destined for the same user. Thereafter, themethod 700 proceeds to step 750, where the eNB selects a secondTTI-length for transporting the second data. The second TTI-length maybe different than the first TTI-length for various reasons. Forinstance, the first data and the second data may have different latencyrequirements and/or buffer sizes, and/or then user's mobilitycharacteristics may have changed. Next, the method 700 proceeds to step760, where the eNB transmits the second data in a second TTI having thesecond TTI-length.

FIG. 8 illustrates a protocol diagram for a communications sequence 800for adapting the TTI-lengths used for carrying data to a common user.The communications sequence 800 begins when a Data_(—)1 810 destined fora UE 115 is communicated from the backhaul network 130 to the eNB 110.Upon reception, the eNB 110 selects a TTI-length for transporting theData_(—)1 810, which the eNB 110 communicates to the UE 110 via the TTIlength configuration (Data_(—)1) message 820. Thereafter, the eNB 110communicates the Data _(—)1 810 in the DL data transmission (Data_(—)1)830. Thereafter, a Data_(—)2840 destined for a UE 115 is communicatedfrom the backhaul network 130 to the eNB 110. Upon reception, the eNB110 selects a TTI-length for transporting the Data_(—)2840, which theeNB 110 communicates to the UE 110 via the TTI length configuration(Data_(—)2) message 850. Thereafter, the eNB 110 communicates theData_(—)2 840 in the DL data transmission (Data_(—)2) 860. In anembodiment, the DL data transmission (Data_(—)1) 830 and DL datatransmission (Data_(—)2) 860 may be carried in the TTIs having differentTTI lengths. The DL data transmission (Data_(—)1) 830 and DL datatransmission (Data_(—)2) 860 may be communicated in the same, ordifferent, radio frames.

In some embodiments, the TTI structure of radio frames may be adapteddynamically, such the TTI length configuration messages/indications areincluded in the Group/UE-specific control channel of each TTI. On onehand, dynamically adapting the TTI structure of radio frames with suchgranularity may provide high degrees of flexibility with respect toTTI-length adaptation. On the other hand, the inclusion of additionalcontrol signaling in the UE/group specific control channel maysignificantly increase overhead in the radio frame, as the UE/groupspecific control channel is communicated relatively frequently (e.g., ineach TTI). To reduce the overhead attributable to TTI-length adaptation,the TTI structure of radio frame may be adapted in a semi-static manner.

FIG. 9 illustrates an embodiment of a DL channel 900 carrying aplurality of variable-length TTIs in a plurality of radio frames910-920. The DL channel 900 may be somewhat similar to the DL channel300, with the exception that the DL channel 900 carries the TTI lengthconfiguration messages/indications in the common control channel, ratherthan the UE-Group specific control channels. This may reduce theoverhead attributable to TTI-length adaptation when high-frequencyadaptation is unnecessary. Furthermore, different TTI-lengths may occupydifferent portions of the DL channel 900 through bandwidth partitioning.Such bandwidth partitioning may depend on the amount of UEs configuredfor a particular TTI length. For example, if there are twice the amountof UEs configured for the short TTI-length than the medium TTI-length,the bandwidth occupied by the short TTI-length may be twice the amountof bandwidth occupied by the medium-TTI length. An advantage of thissemi-static arrangement is that the UEs know the TTI location in timeand bandwidth partitioning by virtue of the aforementioned configurationmessages/indications, and consequently the UEs only need to look for itsUE/Group specific control channels in the time-frequency regionscorresponding to the particular TTI-length. Hence, rather than having tosearch for the entire bandwidth and every atomic interval for itsUE/Group specific control channels, this arrangement reduces the controlchannel decoding complexity of a UE.

A further alternative for reducing overhead is to perform TTI-lengthadaptation in radio frames that have a static TTI structure. In thiscontext, radio frames having a static structure comprise a variety ofTTI-lengths with which to schedule users, but the ratio and placement ofTTIs is fixed such that TTI-length does not change from one radio frameto another. FIG. 10 illustrates a downlink channel 1000 forcommunicating radio frames 1010-1020 having a static TTI structure.Notably, the radio frame 1010 and 1020 have identical TTI structuressuch that the placement/ratio of the short, medium, and long TTIs doesnot change from one radio frame to another. Hence, TTI-length adaptationis accomplished in the downlink channel 1000 through selectivescheduling (e.g., scheduling users to different TTI-lengths), ratherthan by adapting the TTI structure of the radio frames 1010-1020.Similarly, TTI-length adaptation can be achieved via carrieraggregation. FIG. 11 illustrates a downlink channel 1000 for achievingTTI-length adaptation via carrier aggregation. As shown, mid-length TTIsare carried in the frequency band 1110, short-length TTIs are carried inthe frequency band 1120, and long-length TTIs are carried in thefrequency band 1130. Like the fixed-frame structure of the downlinkchannel 1000, TTI-length adaptation is accomplished in the downlinkchannel 1100 through selective scheduling (e.g., scheduling users todifferent TTI-lengths).

FIG. 12 illustrates a block diagram of an embodiment of a communicationsdevice 1200, which may be implemented as one or more devices (e.g., UEs,eNBs, etc.) discussed above. The communications device 1200 may includea processor 1204, a memory 1206, a cellular interface 1210, asupplemental wireless interface 1212, and a supplemental interface 1214,which may (or may not) be arranged as shown in FIG. 12. The processor1204 may be any component capable of performing computations and/orother processing related tasks, and the memory 1206 may be any component(volatile, non-volatile, or otherwise) capable of storing programmingand/or instructions for the processor 1204. In embodiments, the memory1206 is non-transitory. The cellular interface 1210 may be any componentor collection of components that allows the communications device 1200to communicate using a cellular signal, and may be used to receiveand/or transmit information over a cellular connection of a cellularnetwork. The supplemental wireless interface 1212 may be any componentor collection of components that allows the communications device 1200to communicate via a non-cellular wireless protocol, such as a Wi-Fi orBluetooth protocol, or a control protocol. The supplemental interface1214 may be any component or collection of components that allows thecommunications device 1200 to communicate via a supplemental protocol,including wire-line protocols.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

What is claimed:
 1. A method of communicating data in a wirelesschannel, the method comprising: receiving, by an access point(AP), atleast a first data and a second data from a network; and transmitting,by the AP, the first data in a first transmission time interval (TTI)and the second data in a second TTI of a downlink channel in accordancewith a TTI configuration, wherein the first TTI and the second TTI havedifferent TTI lengths based on the TTI configuration.
 2. The method ofclaim 1, wherein the first TTI and the second TTI have different fixedTTI lengths based on the TTI configuration.
 3. The method of claim 1,wherein the TTI lengths of the first TTI and the second TTI aredetermined based on characteristics of the first data and the seconddata, respectively, according to the TTI configuration.
 4. The method ofclaim 3, wherein the TTI lengths of the first TTI and the second TTI aredetermined based on a buffer size associated with the first data and thesecond data, respectively.
 5. The method of claim 3, wherein the TTIlengths of the first TTI and the second TTI are determined based on alatency requirement of the first data and the second data, respectively.6. The method of claim 1, wherein the first data is transmitted to afirst user equipment (UE) and the second data is transmitted to a secondUE, or the first data and second data are transmitted to the first UE;wherein the TTI lengths of the first TTI and the second TTI aredetermined based on characteristics of the first UE and the second UE,respectively, according to the TTI configuration.
 7. The method of claim6, wherein the TTI lengths of the first TTI and the second TTI aredetermined based on mobility characteristics of the first UE and thesecond UE, respectively, according to the TTI configuration.
 8. Anaccess point(AP) comprising: a processor; and a non-transitory computerreadable storage medium storing programming for execution by theprocessor, the programming including instructions to: receive at least afirst data and a second data from a network; and transmit the first datain a first transmission time interval (TTI) and the second data in asecond TTI of a downlink channel in accordance with a TTI configuration,wherein the first TTI and the second TTI have different TTI lengthsbased on the TTI configuration.
 9. The AP of claim 8, wherein the firstTTI and the second TTI have different fixed TTI lengths based on the TTIconfiguration.
 10. The AP of claim 8, wherein the TTI lengths of thefirst TTI and the second TTI are determined based on characteristics ofthe first data and the second data, respectively, according to the TTIconfiguration.
 11. The AP of claim 10, wherein the TTI lengths of thefirst TTI and the second TTI are determined based on a latencyrequirement of the first data and the second data, respectively.
 12. TheAP of claim 10, wherein the TTI lengths of the first TTI and the secondTTI are determined based on a buffer size associated with the first dataand the second data, respectively.
 13. The AP of claim 8, wherein thefirst data is transmitted to a first user equipment (UE) and the seconddata is transmitted to a second UE, or the first data and second dataare transmitted to the first UE, wherein the TTI lengths of the firstTTI and the second TTI are determined based on characteristics of thefirst UE and the second UE, respectively, according to the TTIconfiguration.
 14. The AP of claim 13, wherein the TTI lengths of thefirst TTI and the second TTI are dynamically determined based onmobility characteristics of the first UE and the second UE,respectively, according to the TTI configuration.
 15. A method forcommunicating data in a wireless channel, the method comprising:receiving, by an access point (AP), at least a first data from anetwork; and transmitting, by the AP, the at least the first data in afirst transmission time interval (TTI) of a downlink channel, whereinthe downlink channel has at least a first radio frame, the first radioframe comprising at least the first TTI and a second TTI communicated ina common data channel according to a TTI configuration, and wherein thefirst TTI and the second TTI have different TTI lengths based on the TTIconfiguration.
 16. The method of claim 15, wherein the first TTI and thesecond TTI have different fixed TTI lengths based on the TTIconfiguration.
 17. The method of claim 15, wherein the TTI lengths ofthe first TTI and the second TTI are determined based on characteristicsof data carried by the first TTI and the second TTI, respectively,according to the TTI configuration.
 18. The method of claim 17, whereinthe TTI lengths of the first TTI and the second TTI are determined basedon a buffer size associated with data carried by the first TTI and thesecond TTI, respectively.
 19. The method of claim 17, wherein the TTIlengths of the first TTI and the second TTI are determined based on alatency requirement of data carried by the first TTI and the second TTI,respectively.
 20. The method of claim 15, wherein the TTI lengths of thefirst TTI and the second TTI are determined based on a mobilitycharacteristic of user equipments (UEs) receiving data carried by thefirst TTI and the second TTI, respectively.
 21. An access point(AP)comprising: a processor; and a non-transitory computer readable storagemedium storing programming for execution by the processor, theprogramming including instructions to: receive at least a first datafrom a network; and transmit the at least the first data in a firsttransmission time interval (TTI) of a downlink channel, wherein thedownlink channel has at least a first radio frame, the first radio framecomprising at least the first TTI and a second TTI communicated in acommon data channel according to a TTI configuration, and wherein thefirst TTI and the second TTI have different TTI lengths based on the TTIconfiguration.